The present invention relates in general terms to the field of production of semiconductor lasers based on III–V semiconductor material, in particular semiconductor lasers of this type which are produced by being cleaved from a larger semiconductor crystal (bar) and accordingly have cleaved edges. In particular, in this context the invention relates to a method for passivating these cleaved edges or, in general terms, the resonator end faces of semiconductor lasers.
First of all, the conventional production of the semiconductor lasers will be explained in more detail with reference to FIGS. 1a, b. Production takes place substantially in three steps. First of all, a laser structure is produced by epitaxial coating of a semiconductor crystal. Secondly, the laser structure is processed by lithography and provided with contact metal. Thirdly, the laser mirrors are produced by cleaving the crystal along the [110] crystal axes (in the case of polar compound semiconductors). This cleaving operation also defines the resonator length of the laser, which is limited by two opposite cleaved surfaces 5 which serve as mirrors. The cleaving operation produces a semiconductor strip (laser bar) which includes a large number of laser diodes. The laser diodes can comprise prestructured strips 4 which are arranged next to one another on the laser bar (FIG. 2a). The individual laser diodes 6 can be cleaved from the laser bar.
Methods for passivating the cleaved edge of semiconductor laser diodes that deviate from this invention are known (for example U.S. Pat. No. 4,656,638, U.S. Pat. No. 5,063,173, U.S. Pat. No. 5,665,637, U.S. Pat. No. 5,933,705). The benefit of the passivation manifests itself in a significant increase in the service life of the laser diode at high optical outputs. The effect of the passivation is attributable to the problem that the surface of semiconductor crystals has defects which originate from unsaturated surface bonds and from oxides and impurities which are formed in the atmosphere. While the laser diode is operating, these surface defects lead to absorption of the laser light at the cleaved edge, which simultaneously serves as a mirror surface of the laser. Consequently, the mirror surface is heated, with the result that, at a high optical power density, sudden destruction of the laser diode results. This effect is described in the literature as catastrophic optical mirror damage (journal “APPLIED PHYSICS LETTERS”, 1998, volume 73 (9), pages 1182–1184). The density of the surface defects is reduced by passivation as a result of partial saturation of the surface bonds (“JOURNAL OF APPLIED PHYSICS”, 2000, volume 87 (11), pages 7838–7844). At the same time, oxidation and contamination are prevented (U.S. Pat. No. 5,063,173).
According to the prior art, the passivation of the cleaved edge of semiconductor laser diodes is carried out using thin films consisting of the elements Si, Ge or Sb (U.S. Pat. No. 5,063,173) or the II–VI compound semiconductors (U.S. Pat. No. 5,665,637), such as ZnSe, and other materials (e.g. U.S. Pat. No. 4,656,638, EP 845839). Hitherto, the deposition of the passivation layer has been carried out only for those semiconductor laser diodes which have already been provided with contact metal prior to the passivation. Therefore, the previous methods have been subject to considerable limitations with regard to the choice of materials and parameters. This is essentially attributable to two reasons. Firstly, the high substrate temperature required for optimum deposition (e.g. 500–600° C. for InGaAsP by means of molecular beam epitaxy) leads to irreparable destruction of the contact resistance as a result of diffusion of the contact metal. Secondly, the contact metal, by outgasing contaminates the reactor in which the epitaxy is performed. These contaminants then reach the cleaved edge.
Consequently, the choice of methods and materials for the passivation was hitherto limited to those for which the temperature of the substrate during the deposition is significantly lower than the temperature for the diffusion of the contact metal (350–400° C.) (U.S. Pat. No. 5,933,705). By way of example, the elements Si, Ge, Sb and other materials which are suitable for the passivation are deposited at room temperature. The optimum substrate temperature for the epitaxial growth of ZnSe on GaAs-based semiconductor substrates is 250° C. journal “APPLIED PHYSICS A”, 1999, volume 68, pages 627–629). Therefore, ZnSE layers which have been deposited at a low temperature are used for the passivation.
The methods for passivating cleaved edges which have been employed in accordance with the prior art do not offer complete protection against catastrophic optical mirror damage (journal “ELECTRONICS LETTERS”, 1999, volume 35 (6), pages 506–508). The passivated high-power laser diodes which are used as pump lasers for optical fibre amplifiers suffer catastrophic optical mirror damage at a very high optical power density (e.g. a few 108W/cm2). This is attributed to two main drawbacks of the known methods. Firstly, the passivated cleaved edges still have a high density of absorption centres. The absorption centres are attributable to defects which are able to absorb the light emitted by the laser and are formed, for example, by unsaturated surface bonds, punctiform defects, dislocations, oxides and impurities. Secondly, the materials are structurally unstable over prolonged periods.
The drawback of a cleaved edge of a GaAs substrate which has been passivated with Si resides in the fact that the density of the absorption centres is high (approximately 1010 cm−2eV−1) (journal “VACUUM”, 2000, volume 57, pages 111–120). A further drawback results from the fact that layers which consist of the elements Si, Ge or Sb on III–V semiconductor substrates (e.g. GaAs) are polycrystalline or amorphous and can therefore recrystallize. This structural instability can lead to the formation of surface defects, which in turn contribute to catastrophic optical damage.
The drawback of passivation of the cleaved edge with ZnSe is attributable to a reduced crystal quality (journal “APPLIED PHYSICS A”, 1999, volume 68, pages 627–629). The ZnSe layers which have been deposited on (011)-GaAs by means of epitaxy have crystal defects with a high density and surface roughness. The laser diodes which are based on the crystal growth of ZnSe on (011)-GaAs are known to degrade rapidly, demonstrating the poor crystal quality of ZnSe on (011)-GaAs. Other materials and methods which are used for passivation in the prior art also have drawbacks. Laser diodes which have been passivated in this way suffer catastrophic optical mirror damage even at lower loads than the laser diodes which are passivated with Si or ZnSe (e.g. journal “ELECTRONICS LETTERS”, 1999, volume 35 (6), pages 506–508).